Proper development of the immune system involves a series of life-and-death decisions that are mediated through well-orchestrated coordination of signaling pathways, including that of the apoptosis-inducing cell death receptor Fas. Apoptotic signaling mediated by Fas eliminates autoreactive and controls the expansion of activated lymphocytes. Not surprisingly, studies have shown that alterations of Fas receptor signaling pathways promote the development of autoimmune lymphoproliferative syndrome and hematopoietic cancers. Hematopoietic cancer cells commonly express adequate levels of wild-type Fas, but this Fas is often activation-resistant for unknown reasons. Such limitations to our understanding of the mechanisms that cause resistance to apoptosis in leukemias and lymphomas stand in the way of advances in the treatment of these cancers. On the basis of our finding of a virally encoded inhibitor of Fas, we anticipated that cancer cells express similar inhibitors of Fas. Indeed, our search has identified nucleolin as a potential binding inhibitor of Fas. We found that overexpressed nucleolin bound to Fas in lymphoma cells and protected against Fas apoptosis in culture and in mice. Nucleolin knockdown sensitizes the cells to Fas apoptosis. We hypothesize that cell-surface nucleolin interacts directly with Fas and inhibits Fas-mediated apoptosis and that nucleolin represents a feasible target for cancer therapy. To test this hypothesis, we will identify a possible mechanism of nucleolin-mediated regulation of Fas apoptosis in cancer by analyzing Fas signaling in the presence and absence of nucleolin. We will define the Fas-interacting domain on nucleolin and produce mutants that lack binding to Fas. We will express these deletion mutants in mice to determine whether Fas nonbinding nucleolin mutants protect against cell death induced by Fas activation. We will characterize molecular interactions of nucleolin with Fas in cells and mice. This analysis will build on the finding the nucleolin offers resistance to Fas apoptosis. The long-term goal of this project is to develop targeted therapies to directly promote cancer cell apoptosis and greatly enhance current cancer therapies.